round off all currency values to four decimal places.

小数点 is referring to the dot.
小数点后保留四位 means to retain 4 decimal places after the dot.



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Note added at 25 mins (2011-04-15 15:12:29 GMT)
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the dot = the decimal point

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Note added at 27 mins (2011-04-15 15:14:34 GMT)
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So, if it is a whole number (no decimal places), you still write it with 4 decimal places. E.g. 10.0000 or 15.0000
Write all currency values using four decimal places.

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Note added at 38 mins (2011-04-15 15:24:40 GMT)
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四位 is not equal to four blank spaces, but equal to four decimal places. You have to maintain 4 decimal places after the decimal point whenever you present (write out) a currency value (amount of money) on the form.

This is the meaning.

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Note added at 1 hr (2011-04-15 16:03:08 GMT)
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If you ask me, I'd translate it as follows:
表中所有填写的金额数小数点后保留四位。
Fill out all currency values in the form using four decimal places of accuracy.
OR
Round off all currency values in the form to four decimal places.

Or some other phrasing along this line, as long as it is clear to the reader that he/she has to fill out currency values (amount of money) using 4 decimal places.